IEEE - ICSCN 2007, MIT Campus, Anna University, Chennai, India. Feb. 22-24, 2007. pp.204-207.
Design and Development of Elevation Independent Tracking System
for the Remote Sensing Satellite Data Acquisition
T.C. Sarma, C.V. Srinivas and A. Venugopal
Abstract: Remote sensing satellites are of the class of LEO II. ELEVATION AND AZIMUTH TRACKING LIMITATIONS
and hence visible to any ground station for a short period
of time around 5 to 15 minutes in a given orbit. They also The spherical geometry coverage offered by an elevation-
move with velocities greater than 6 to 8 km/sec. The over-azimuth positioner has several advantages over other
azimuth velocity is a function of CosEL and hence as the positioner geometries, including as a few, mechanical
elevation angle approaches 90deg (>87.5deg) the velocity
simplicity, full hemispherical coverage, ease of operator
control, and direct relationships with coordinate systems
requirements cross the system limits/ specifications. traditionally employed in navigational and scientific fields.
Hence a model has been developed and validated to make Since any positioner is a mechanical device, some constraints
it functional so that Elevation independent data will be placed on the freedom of axis motion by the
acquisition can take place with certain considerations. positioner's physical construction. Since we are concerned
The paper deals with one such model developed and used here primarily with the behavior of the positioner around the
for the remote sensing satellites. zenith position, it can be assumed that the target trajectory is
approximately rectilinear through the small special angular
I. INTRODUCTION range to be investigated with little loss of generality. The
linear orbit approximation (LOA) is adopted throughout.
For the Remote Sensing Satellite Data Acquisition These equations relate the elevation and azimuth angles of
Systems, the satellite trajectory is derived from the given the pedestal to the trajectory of the target as a function of
state vectors of satellite for every satellite pass that is to be time while Assuming the target trajectory is parallel to the x-
handled. In order to track a particular satellite for a given axis of a Cartesian system defined as with the z-axis directed
orbit the essential elements are the satellite trajectory along the Earth centered radial direction,
information derived as above containing the time, the ground
station antenna elevation and azimuth angles, time Az = arc tan 7hicot(EL)
Acquisition of Satellite (AOS), Loss of Satellite (LOS), vt*(t-to)
Satellite Orbit, Path No. and other characteristics of the
satellite. The details of AZ and EL angles to be derived at EL = arc tan [ Vt *( +cot2 ELm } -1/2
regular intervals of time say 20 to 25 seconds along with where - h
UTC. The normal mode of operation for the system is to Vt: Target Velocity
track the satellite in auto track or programme track. For the H Height of Satellite
satellite passes having the elevation angles beyond 87.50 to
900 azimuth velocity requirement becomes almost infinite as to is the time origin at which the elevation angle is
velocity vector of AZ is function of CosEL which cannot be maximum
supported by any system. Therefore the tracking in azimuth ELm: is maximum elevation angle that will be achieved
direction lags for the above angles and hence looses the during the pass
satellite tracking when the lagging angle exceeds the antenna These are derived from the angular rotation matrices of
beam width. Therefore these high elevation passes were the transformations to the spherical coordinates of elevation
analysed and developed a model after simulations thorough and Azimuth which are the transformations between the
computer analysis for error free functionality. Cartesian System and the associated spherical elevation-over-
azimuth system of the positioner.
The required pedestal axis velocities can be found by
differentiation:
Data Archival and Realtime Systems Division, National Remote
Sensing Agency, Govt. of India, Dept. of Space, Balanagar, AZ = d AZ = -hIcotELcos2 EL
Hyderabad- 500 037 Dt vt*(t-to7
Email: sarma tc dWnrsa.gov.in, sarma tc dWyahoo.com
1-4244-0997-7/07/$25.00 ©2007 IEEE 204
IEEE-ICSCN, Feb. 2007
EL = d EL= -h v t0)Cos2 EL band about the true satellite coordinates for -3dB bandwidth.
dt Vttt)+Zt m The most efficient trajectory possible to minimize the
required azimuth velocities is one that tracks the true azimuth
Consider the azimuth velocity: coordinate to a prescribed position and then moves with
Az = -h*cotEL,,1*Vt maximum velocity to the symmetry position at the opposite
[Vt2 *(tto)2+h(cot ELm)] side of the cross over position before reverting to true
coordinate tracking. During this period the tracking is totally
Then the maximum azimuth velocity will obviously occur through programmed mode of operations so that the antenna
at t = to when the elevation angle is maximum, tracking is controlled by the system as per the predetermined
model. Even through an auto tracking system may still lose
So that track of the satellite as a result of velocity limitations and
Az max: -Vt signal degradation, the programme controlled need only to
h*cot(ELm) remain within ± 0.70 of the pointing error of the target to
receive the acceptable data. This required around 20 to 22deg
This equation exhibits the fact that the transformation sec for a minimum speed programmed trajectory. The
requires an infinite azimuth velocity to track a direct analysis was done for IRS series of satellites where the
overhead pass (ELm = 900). pedestal is assumed to within 0.7deg cone during zenith
passes result in almost 6db signal degradation. Based on the
III. SATELLITE TRACKING peak elevation angle during the trajectory deviation, overhead
pass is detected. The model developed provides a method of
The very high azimuth velocities required to precisely control for the antenna in the programmed mode to augment
follow a satellite in a high elevation pass cause limitations on the auto tracking during near overhead and zenith passes. The
the maximum elevation angle pass that a positioner is able to flow chart of the S/W is as given below.
effectively track. Velocity requirements cannot be met as the
effective pointing error exceeds the geometrical azimuth
connection. Therefore the error exceeds the antenna
beamwidth resulting in loss of satellite. If the beamwidth of
the antenna is large or the pedestal velocities are high, it is
possible that the error will not exceed the beamwidth
resulting in tracking the satellite. This concept has been used
in developing the model for a program track system to handle
these categories of satellite passes (visibilities).
IV. THEORY OF THE MODEL DEVELOPED
Along with derivation of the satellites Trajectory
information, it is possible to derive the velocity requirements
of the satellite passes having elevation grater than 870.
During the computer simulation the following were taken
into account and observed for zenith passes (a) very high
Azimuth velocities are required during the overhead pass (b)
Elevation velocities are very small during the overhead pass
(c) Auto tracking is lost when the Azimuth pointing direction
lags the satellite position by an angle exceeding the antenna
beamwidth (d)This lag is a result of maximum azimuth
velocity limitation of the pedestal. It means sufficiently high
azimuth velocity will allow the position to track within the Fig. 1. Simplified Software Flow
antenna bandwidth and receive the pass. For small
beamwidths, however, this becomes an unrealistic V. RESULTS
requirement. For a given antenna beam width for a 10 mtr.
Antenna, based on the different elevation passes, family of The program track system has been fine tuned to work
azimuth trajectories could be constructed that fall within the close to auto track resulting in the least possible errors both
beam width for overhead pass. This plot should limit the in Azimuth and Elevation as per the profiles given below:
205
Design and Development ofElevation Independent Tracking System for the Remote Sensing Satellite Data Acquisition
The predicted and the actuals both in Az and EL match very
close. It took sometime to model behaviour of the system.
The results are excellent and working for all remote sensing
satellites.
Fig. 5. Zenith Pass Velocity Profile
Fig. 2. Tracking Profile Azimuth
-
Fig. 6. Zenith Pass Acceleration Profile
Azimuth Rate
Elevation Rate
2.50E+01
0 2.OOE+01
Fig. 3. Trajectory Profile Elevation
- 0~
0 1.50E+01
The zenith passes were handled by the system very 0
1.OOE+01
efficiently without losing the data with a tolerable 5.OOE+00O
deterioration in the data quality. The zenith pass azimuth O.OOE+00O
tracking angle profiles, velocity profiles and acceleration -5.OOE+00
profiles are as follows: The model was fine tuned with more Time In lOOs MilliSeconds
number of observations for each satellite to result in a stable
reliable system. Fig. 7. AZ EL Rate Profile for MAXEL 88.04 Degrees
Azimulth Rate
Elevation Rate
5.OOE+01
, 4.OOE+01
32.OOE+01
1.OOE+01
,u 2.OOE+01
.2
0. 0E +00 --------
-1.00E+01
Time In 100s MilliSeconds
TN SECONDS
TIME
Fig. 8. AZ EL Rate Profile for MAXEL 89.0 Degrees
Fig. 4. Zenith Pass Tracking Angle Profile
206
IEEE-ICSCN, Feb. 2007
The azimuth velocity requirements increase with the VI. CONCLUSION
increase in the elevation angle and crosses the limits around
the peak elevation beyond 880. This system behaviour around With the concepts of system on chip (SOC) and DSP
the peak elevation is plotted as below which is very important implementation in programmable logics, the model
for the modelling. implementation became easier. Further work is in progress to
implement Lab on Chip (LOC) concept for remote
management of the system through USB.
Azimuth Rate
Elevation Rate
REFERENCES
3.OOE+02
X 2.50E+02 [1] R.Giubilis and S.Badessi, An antenna auto tracking system for
o 2.OOE+02 Ka Band inter datellite links, 14th International Commn. Sat.
m -
1.50E+02 Syst. Conf, AIAA Washington, March 22 - 26, 1992.
D 1.OOE+02 [2] G.J.Hawkins, D.J.Edwards and J.P.McGeehan, "Tracking
c 5.OOE+01 Systems for satellite Comminications, TEEE Proc, Part F, (5)
5O.OOE+001
1988
-5.OOE+01
Time In 1 OOs MilliSeconds
[3] W.B.Davrnport and W.L.Root, An Introduction to theory of
Random signals and noise, New York, Mc Graw Hill.
Fig. 9. AZ EL Rate Profile for MAXEL 89.83 Degrees [4] A.Ritorto and M.Porfiri, Control loop analysis and
performance report, Alenia Spazio Tech. Note ART/APS-
The high elevation passes that could not be handled by 4235, Sept 1994.
auto track are controlled by the zenith controller as per the
model, few seconds prior to peak elevation and keeps the [5] J.K.Holmes, Coherent Speech Spectrum Systems, New York,
control for the symmetry and the other side of the peal Wiley Sons Ltd, 1982.
elevation. As the model tested for different satellite, it could
handle all the passes without failure.
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